Brave Lucina

The game is quickly approaching a state where you cannot hope to survive any kind of encounter without outright striking down anything that looks in your general direction. In this scenario, the most convenient kind of support you can build Brave Lucina as is what you might call an "ATK donator."

For optimal units to receive support, you need only consider the usual suspects: Brave Ike, Altina, and penalty-immune units, to name a few. Units with dual-phase double strikes such as Laslow, Keaton, and Reinhardt (sword) are also great options. These days, one of your stongest options will be Winter Altina, but don't be afraid to use Brave Hector instead if you literally have nothing.

By using double Atk Drives, you can give the supported allies up to 9 ATK. Joint Atk Drive and an ally support can give you a further +3, for up to +12 under ideal conditions. With so much additional attack, specials with lifesteal such as Sol and Aether will become much more useful, and units with double strikes will stand to gain effectively twice as much damage.

Regarding other skill choices: Use Shove over Smite to avoid pushing the supported unit too far away.

While the B-Slot is generally considered a free slot, the mobility creep in Aether Raids is so toxic that you will likely get a surprising amount of value from Escape Route. If you are clever with another ally's Smite, you can potentially turn a Bolt Trap into your own mobility advantage.

Use Death Blow 3 to leverage Lucina's dragon effectiveness. Giving her anything more valuable than that is nothing short of luxurious. In this scenario, you would ideally prefer an ATK asset, but this is otherwise not a deal breaker when you are considering what is essentially the holy grail of support units.

Lastly, the special skill is kind of up in the air, more so than your B-slot skill. You will rarely, if ever, get into many situations where Brave Lucina's native Aether will be available to use, so it's recommend to downgrade to any lower cooldown specials such as Luna or Glimmer.

Modern AR Offense build.

Brave Lucina's support abilities are well known at this point, but there are new skills she can use that can augment her ability to help other units break otherwise difficult Aether Raids maps.

The core addition is Even Pulse Tie, which inflicts Special Cooldown count+2 on a foe with the lowest HP among those with their special ready at the start of each even-numbered turn. This is a critical tool to combat Infantry Pulse defensive teams as well as Ophelia after they charge up on turn 1. Brave Lucina's HP may not be the highest, but being a melee unit has afforded her just enough HP to make use of it against most range 2 threats in this regard.

Weapon Skill: Geirskogul (+Eff)
Geirskogul in its refined state provides incredible support in not only stats but also the Breath effect for added Special cooldown charge rate to allies within 2 spaces.

Assist Skill: Any Positional Assist (Alternate: Any Rally+ or Rally Up+ skills / Harsh Command+ / Reciprocal Aid)
Actual skill selection is up to personal preference.

Special Skill: Moonbow
Actual skill selection is up to personal preference. Moonbow is the common choice for combat, which Lucina is more than capable of when she isn't busy supporting other units.

A Skill: Fury 4 (Alternate: Life and Death 4 / Fort. Def/Res 3)
This skill is used to try and absorb Chill effects from enemies and Bright/Dark Shrines. Life and Death 4 and Fort. Def/Res 3 can also be used to narrow it down Atk/Spd and Def/Res, respectively; be mindful of the stats of her allies before slotting these skills to ensure they will hit her, however, especially with Fort. Def/Res 3 as her base bulk isn't particularly high.

B Skill: Even Pulse Tie 3 (Alternate: Sudden Panic 3 / Any Ruse Skill)
Even Pulse Tie 3 is her core addition, as mentioned. Other options that can be explored are Sudden Panic 3 to combat bonuses, and Ruse skills to apply substantial debuffs and inflict Guard on foes to seize Special advantage against bulkier teams.

C Skill: Joint Drive Atk (Alternate: Any Joint Drive skill / Close Guard 3 / Distant Guard 3 / Any Tactic skill / Any Wave skill)
Any skill that can provide further support to units in her range will due. Joint Drive skills will generally provide her with the most value in scenarios where she needs to fight herself, however.

Seal skill: Drive Atk 2 (Alternate: Any Drive skill / Close Guard 3 / Distant Guard 3 / Any Tactic skill / Any Wave skill)
Same reasoning as C skill.

First and foremost, she is built for support, but can fight to kill red/dragons, and also can be an emergency hero if the main hero has been killed.

This specific build is to get Altina to 75 atk

NOTE: This build will be updated when they decide to put Distant Guard 3 in the Sacred Seal slot. I...honestly don't know why they haven't done it yet if fehstuff of all sites has it done already.
ALSO NOTE: This build is taking into account that the Brave!Lucina has a Def boon and is at +10 merges and +10 Dragonflowers, as well as having an S-Rank Summoner Support. Without counting bonuses, her statline is 55 HP, 58 Atk, 44 Spd, 42 Def, 27 Res.

In my opinion, I think this is Brave!Lucina's best Support build with Geirskogul (at this time). Below are the pros and cons that I know of with this build so far.

Pros:
1) Anti-dragon effectiveness. With Geirskogul's refinement she gains this effect, and with Ruptured Sky she can increase her damage to them even further very quickly. Atk/Spd Push 4 increases her Atk/Spd to 65/51, making her offensive presence just as dangerous as her supportive presence (especially to dragons).
2) Quality tankiness. Thanks to her high Spd combining with Atk/Spd Push 4, Close Call 3 has 51 Spd to work with, meaning that any foe with 41 or less Spd will deal 40% less damage to her (discounting any Spd debuffs they might inflict).
3) High offensive presence. Like it was explained in Pros 1, her offensive presence is pretty high. With 65/51 Atk/Spd, she doubles anything with 46 or less Spd, and with Ruptured Sky at 2 cooldown she'll always be able to trigger it under normal circumstances. Also, due to Close Call, any unit with 41 or less Spd that has the Atk to deal real damage to her despite the damage reduction will inherently take more damage from Ruptured Sky.
4) God-Tier Support. This is really what you're here for. I've been talking about her fighting others by herself, but what you're really using this build for is her role as a Support. Her refined Geirskogul gives All+3 and super-charging during the Enemy Phase to allies within 2 spaces, and with both Close and Distant Guard she also provides those allies with a dual phase Def/Res+4 during combat (effectively giving her Drive-distance allies Def/Res+7 during the Enemy Phase).
5) Great positional utility. With Close Call having Hit and Run built into it, and her Geirskogul/dual Guards having their generous Drive range, it's quite easy to set herself and her team up both offensively and defensively. You can very easily switch Reciprocal Aid for Reposition and the like; the reason I won't here is purely for healing her HP for Atk/Spd Push 4, but this CAN be easily solved by putting a Healer or pseudo-Healer on her team.
6) Godly dueling presence due to Close Call. This is more of a continuation of both Pros 2 and 3, but I felt it had to be said. She can actually out-duel an unmerged +Atk Winter!Ephraim that's initiating on her with his natural skills if she has Ruptured Sky charged by her first attack. Because of Close Call, she can actually tank his Dragon Fang (which is even more powerful due to him having 71 Atk during combat), and only take 38 damage from it, and on her counterattack he'll take 53 damage (if she activates Ruptured Sky at this time) and get OHKO'd. Yes, he's umerged...but this is a Support build and she's capable of out-dueling the guy with the highest possible natural Atk in the game so far. I think that's worth something.

Cons:
1) No Distant Counter. Atk/Spd Push 4 is necessary to maximize the effectiveness of both Ruptured Sky and Close Call, but this leaves her without the ability to counterattack against ranged foes during the Enemy Phase. This leaves her especially vulnerable to Tomes and Wrathful Staffers, and also daggers to an extent.
2) Low magical bulk. Close Call can only mitigate so much damage, and 82 magical bulk isn't all that much when you have so many great magic-based Nukes present in the game. Even some of the very dragons she one-shots have a great chance at destroying her during the Enemy Phase if you aren't careful with her positioning beforehand.
3) No defense against debuffs or defensive Bonus Doublers. She's capable of taking advantage of offensive Bonus Doublers due to Ruptured Sky, but defensive strategies leave her doing less than she wants. Since she uses Close Call in this build instead of a Lull or Dull skill, she has to make do with brute force or just simply leaving it to her allies to finish the job. On the debuffs side, Close Call's own effect falls off when faced with Spd debuffs (though thankfully this is largely mitigated by Atk/Spd Push 4 being in-combat and not a field bonus that can be Panic'd). Res debuffs are also a massive concern due to her sub-optimal magical bulk.
4) Green Tanks. Even gods can only do so much, and Brave!Lucina (despite not actually being a goddess) is no different. If she faces a foe that she just can't deal damage to regardless of her offensive statline, she'll need to rely on her allies to take them down for her. And though the synergy of Close Call and Ruptured Sky is really amazing, tanks with lower Atk make Ruptured Sky have less usage than she needs against said foes. Having a Red ally that can easily deal with tanky Green foes, magic or not, is a must for her.
5) Geirskogul's reliance on physical allies. This is old news with anyone using Brave!Lucina, but I felt it needed to be mentioned. While her magical allies will be affected by the dual Guards, they won't be able to be affected by Geirskogul's effect. If you're going to have her work with a magical ally, make sure they can work fairly independently from the rest of the team (or at least independently from her).

There are probably a lot more pros and cons than what I've listed, and I'll be coming back anyhow once Gamepress adds Distant Guard to the list of Sacred Seals, so expect updates. Note, however, that I've permanently left the Discord server and will NOT be available for advice because the mods there are trash and their rules are worse than trash.
If anyone wants to discuss Brave!Lucina with me, I go by the same name on here as I do on SerenesForest. I don't think I can actually set up a topic dedicated to Brave!Lucina on there without it getting removed, but feel free to message me there regardless.

Geirskögul (+Eff) +3 HP +16 Mt.
Effective against dragon foes. Grants Def+3. Grants Atk/Spd/Def/Res+3 to sword, lance, axe, bow, dagger, or beast allies within 2 spaces during combat, and if foe initiates combat against those allies, grants Special cooldown charge +1 per attack to those allies. (Only highest value applied. Does not stack.)